As a membrane module used for a membrane separation device, there has been known a spiral wound type membrane module in which separation membranes are wound around the outer circumference of a water collection pipe.
FIG. 5 is a perspective view, partly exploded, showing a structure of a conventional spiral wound type membrane module.
A plurality of envelope-shaped membranes 2 having a shape of an envelope are wound onto a water collection pipe 1 with mesh spacers (outer spacers) 3 being interposed between the membranes.
The water collection pipe 1 is provided with a slit-like opening for allowing communication between the outside and the inside of the pipe. Inside of the membrane 2 formed in an envelope shape, communicates with the inside of the water collection pipe 1. Inserted into each envelope-shaped membrane 2 is an inside spacer 4 including mesh-like spacer, for forming a channel inside the membrane, so that the inside of the envelope-shaped membrane (hereinafter, sometimes referred to as just "membrane") 2 constitutes a permeated water channel.
The opposite ends of a roll 5 of the membranes 2 are closed with a top ring 6 and an end ring 7 at its edges and brine seals 8 are fitted around the top ring 6 and the end ring 7.
Feed water (raw water) flows from the front end of the membrane roll 5 into a feed water channel(raw water channel) between the membranes 2 and further flows in the longitudinal direction of the membrane roll 5. Nonpermeated water is outputted from outlets of the rear end of the membrane roll 5. While the water passes in the feed water channels, the water permeates through the membranes 2 and flows into the pipe 1 and is taken out from the rear end of the pipe 1 as permeated water.
The spiral wound type membrane module is easily clogged and it is difficult to operate in such a manner to keep a flux of the permeated water at a high rate for a long period.
The conventional spiral wound type membrane module has also the following detects:
1) The water collection pipe 1 must have larger diameter to obtain a great flux within the pipe 1. However, this also makes the diameter of the spiral wound type membrane module larger.
2) The permeated water flows in each membrane spirally to the water collection pipe 1, so that a flow resistance increases in the membrane. A flow resistance is also high around the slit-like openings through which the permeated water flows from the inside of the membrane 2 into the water collection pipe 1.
3) A volume of the feed water flowing in the feed water channels decreases as the feed water flows to the outlet by permeation into the membranes, and a flow velocity of the feed water becomes small in a downstream region of the feed water channels whereby the membranes are easy to be fouled therearound.